Bash script - how can I concatenate string with a variable and use as message for MQTT

Question

I am trying to create a script in bash in order to republish 1 MQTT message for 3 virtual sensors in Domoticz. I am running into a trouble while creating message part for mossquito_pub command

#!/bin/bash
# This script subscribes to a MQTT topic using mosquitto_sub.
# On each message received, you can execute whatever you want.

while true  # Keep an infinite loop to reconnect when connection lost/broker unavailable
do
    mosquitto_sub -h "192.168.1.30" -t "tele/tasmota_D875F9/SENSOR" | while read -r payload
    do
        # Here is the callback to execute whenever you receive a message:
        str="${payload}"
        voltage1= echo $str | cut -f 22 -d ','|  tr -dc '0-9'
        echo $voltage1
        mosquitto_pub -h 192.168.1.30 -p 1883 -t 'domoticz/in' -m  '{"idx" : 33, "nvalue" : 0, "svalue" : "$voltage1" }'
        voltage2= echo $str | cut -f 23 -d ','|  tr -dc '0-9'
        echo $voltage2
        mosquitto_pub -h 192.168.1.30 -p 1883 -t 'domoticz/in' -m  '{"idx" : 34, "nvalue" : 0, "svalue" : "$voltage2" }'
        voltage3= echo $str | cut -f 24 -d ','|  tr -dc '0-9'
        echo $voltage3
        mosquitto_pub -h 192.168.1.30 -p 1883 -t 'domoticz/in' -m  '{"idx" : 35, "nvalue" : 0, "svalue" : "$voltage3" }'
    done

    sleep 10  # Wait 10 seconds until reconnection
done # &  # Discomment the & to run in background (but you should rather run THIS script in background)


    sleep 10  # Wait 10 seconds until reconnection
done # &  # Discomment the & to run in background (but you should rather run THIS script in background)

Currently, terminal echo output is:

230
231
233

Message transmitted:

2020-12-01 13:51:27.952 MQTT: Topic: domoticz/in, Message: {"idx" : 33, "nvalue" : 0, "svalue" : "$voltage1" }
2020-12-01 13:51:28.057 MQTT: Topic: domoticz/in, Message: {"idx" : 34, "nvalue" : 0, "svalue" : "$voltage2" }
2020-12-01 13:51:28.161 MQTT: Topic: domoticz/in, Message: {"idx" : 35, "nvalue" : 0, "svalue" : "$voltage3" }

I need variables voltage1, voltage2, voltage3 to be replaced by their values as they are seen in terminal echo output including quotation marks in front and right after.

Please help me.